1. Field of the Invention
The present invention relates to liquid crystal displays, image signal compensation circuits, compensation methods therefor, and electronic apparatuses in which what is referred to as xe2x80x9cburn-inxe2x80x9d is prevented.
2. Description of Related Art
Typically, liquid crystal panels, which perform predetermined display using liquid crystal, have a structure in which the liquid crystal is held between a pair of substrates. These liquid crystal panels can be classified according to their driving systems. For example, an active-matrix-type, in which pixel electrodes are driven by switching elements has on one substrate of the pair of substrates, a plurality of scanning lines and a plurality of data lines which are formed so as to intersect with each other, while insulation is maintained therebetween. A thin film transistor (xe2x80x9cTFTxe2x80x9d), which is an example of a switching element and a pixel electrode form a pair at each intersection. Around a region (display region) in which pixel electrodes are formed, peripheral circuits for driving scanning lines and data lines are provided. On the other substrate, a transparent counter electrode (common electrode) opposed to the pixel electrodes is provided, and the counter electrode is maintained at a predetermined potential. Furthermore, disposed on the opposing surfaces of the two substrates, are alignment layers, rubbed so that a long-axis direction of liquid crystal molecules is continuously twisted between the two substrate at, for example, approximately 90 degrees. On the back surface of each of the two substrates, is disposed a polarizer in accordance with the alignment direction.
Concerning the switching element provided at the intersection of the scanning line and the data line, when a scanning signal (gate signal) applied to the corresponding scanning line becomes an ON-state potential, a connection between the source connected to the data line and the drain connected to the pixel electrode is established. Thus, an image signal supplied to the data line is applied to the pixel electrode, and the potential difference between the potential of the counter electrode and the potential of the image signal is applied to a liquid crystal capacitor formed of the pixel electrode, the counter electrode, and the liquid crystal therebetween. If the switching is turned off, the liquid crystal capacitor maintains the applied potential difference in accordance with the characteristics of the liquid crystal capacitor and a storage capacitor.
If the effective voltage applied across the two electrodes is zero, light which passes between the pixel electrode and the counter electrode is rotated by approximately 90 degrees along the twisting of the liquid crystal molecules. As the effective voltage increases, the liquid crystal molecules tilt toward the electrical field direction, resulting in loss of rotatory polarization. For example, in a transmissive-type liquid crystal display, when polarizers in which polarizing axes are orthogonal to each other in accordance with the alignment direction are formed at the light-incident side and the back side (in normally white mode), and when the effective voltage applied across the two electrodes is zero, light passes between the two electrodes, thereby displaying white (transmissivity becomes high). As the effective voltage applied across the two electrodes increases, light is blocked, and eventually black is displayed (transmissivity becomes low). By supplying scanning signals to the scanning lines and image signals to the data lines with an appropriate timing, the effective voltage in accordance with the gray level can be applied to each liquid crystal capacitor. As a result, gray-scale display in which the gray-level differs for each pixel can be performed.
In principle, a liquid crystal display employs an AC driving system for driving the liquid crystal capacitor in order to prevent deterioration of the liquid crystal, which is caused by application of a direct current (DC) component. An image signal applied to the pixel electrode via the data line is alternately inverted every predetermined period between the positive polarity and the negative polarity on the basis of a predetermined constant potential Vc.
In a switching element, such as a TFT, a phenomenon which is referred to as xe2x80x9ca push-down phenomenonxe2x80x9d occurs. Specifically, as shown in FIG. 6(a), the push-down phenomenon means that, when a scanning signal (gate signal) changes from an ON-state potential Vdd to an OFF-state potential Vss, the potential displacement reduces the potential of the drain (pixel electrode) via a parasitic capacitance between the gate and the drain.
The potential displacement caused by the push-down phenomenon increases as a write potential, namely, a source potential, decreases. When voltages Vgp and Vgn, which correspond to the same gray level, are written towards the positive polarity side and the negative polarity side, potential displacements PD and ND caused by the push-down phenomenon become larger at the negative polarity side.
When light passes between the two substrates, part of the light enters the TFT. Even when the scanning signal becomes an OFF-state potential Vss, thereby entering the off period (holding period), a small leakage current (light current) flows through the TFT. The degree of leakage may differ between the positive polarity writing and the negative polarity writing.
Accordingly, the effective voltage actually applied to the liquid crystal capacitor (which corresponds to portions indicated by oblique lines in FIG. 6(a)) may differ between the positive polarity writing and the negative polarity writing, and hence a DC component is applied to the liquid crystal. When the DC component is applied to the liquid crystal, the liquid crystal deteriorates due to dielectric polarization or the like. As a result, xe2x80x9cburn-inxe2x80x9d occurs.
In view of the foregoing circumstances, it is an object of the present invention to provide a liquid crystal display, an image signal compensation circuit, a compensation method, and an electronic apparatus, in which xe2x80x9cburn-inxe2x80x9d is suppressed.
In order to achieve the foregoing objects, a first aspect of the invention is a liquid crystal display image signal compensation circuit having a plurality of scanning lines, a plurality of data lines, and pixels which correspond to intersections between the scanning lines and the data lines. The pixels comprised of a pixel electrode and opposing electrode between which liquid crystal is sandwiched to form a liquid crystal capacitor, and a switching element which establishes a connection between the corresponding data line and the pixel electrode in accordance with the level of a signal supplied to the corresponding scanning line. The liquid crystal display inverts the polarity of an image signal, which is in synchronization with horizontal scanning and vertical scanning, every predetermined period on the basis of a predetermined constant potential, and supplies the image signal to the pixel electrode through the data line. The image signal compensation circuit that compensates for the image signal for the liquid crystal display includes a compensation-level output unit to output a compensation level which corresponds to the level of the uncompensated image signal; and an adder that adds the compensation level to the uncompensated image signal and outputs the sum as the compensated image signal.
According to this arrangement, the compensation-level output unit outputs a value in consideration of the effects of push-down and light leakage as a compensation level with respect to an image signal at a particular level. Thus, the effective voltage actually applied to the liquid crystal capacitor when a positive-polarized image signal is applied to the pixel electrode, and the effective voltage actually applied to the liquid crystal capacitor when a negative-polarized image signal is applied are approximately the same. As a result, application of a DC component to the liquid crystal capacitor is prevented, thereby suppressing deterioration of the display quality due to burn-in or the like.
According to the present invention, no problem occurs as long as the effective voltage at one polarity eventually becomes equal to the effective voltage at the other polarity. It is thus unnecessary to output compensation levels at both polarities, namely, the positive polarity and the negative polarity. According to the first aspect of the invention, it is preferable that the compensation-level output unit output the compensation level which corresponds to only the level of the image signal at one polarity, and preferably the compensation-level output unit outputs approximately zero as the compensation level which corresponds to the level of the image signal at the other polarity. According to this arrangement, it is only necessary to output a compensation level which corresponds to only one polarity. Thus, the structure is simplified.
In order to simplify the structure, it is preferable that the compensation-level output unit include a table in which the relationship between the level of the uncompensated image signal and the compensation level is stored beforehand.
In this arrangement, preferably the table stores the relationship at two or more points. When the level of the uncompensated image signal is within the range of the stored relationship, it is preferable that the table interpolate and output the compensation level which corresponds to the level of the uncompensated image signal based on the stored relationship. According to this arrangement, it is unnecessary to store compensation levels for image signals at all possible levels, thereby reducing the storage capacity required for the table.
In order to reduce the storage capacity, when the level of the uncompensated image signal is outside the range of the stored relationship, it is preferable that the table estimate and output the compensation level which corresponds to the level of the uncompensated image signal based on the stored relationship. Concerning estimation modes, various modes can be employed, such as a mode of obtaining a compensation level by extrapolation using the correlated points stored in the table, a mode of obtaining a compensation level using an extension of the straight line between the correlated points, and a mode of multiplying the compensation level at the nearest point by a coefficient in accordance with the distance from the nearest point.
In order to achieve the foregoing objects, a second aspect of the invention is an image signal compensation method for a liquid crystal display. The liquid crystal display includes pixels corresponding to intersections between a plurality of scanning lines and a plurality of data lines. The pixels comprised of a pixel electrode and opposing electrode between which liquid crystal is sandwiched to form a liquid crystal capacitor, and a switching element which establishes a connection between the corresponding data line and the pixel electrode in accordance with the level of a signal supplied to the corresponding scanning line. The liquid crystal display inverts the polarity of an image signal, which is in synchronization with horizontal scanning and vertical scanning, every predetermined period on the basis of a predetermined constant potential, and supplies the image signal to the pixel electrode through the data line. The image signal compensation method for compensating for the image signal for the liquid crystal display includes the steps of outputting a compensation level which corresponds to the level of the uncompensated image signal; and adding the compensation level to the uncompensated image signal and outputting the sum as the compensated image signal.
Since the second aspect of the invention is a method corresponding to the first aspect of the invention, in a similar manner, application of a DC component to the liquid crystal capacitor is prevented, thereby suppressing deterioration of the display quality due to burn-in.
According to the liquid crystal display, in a region in which pixels are at intermediate gray levels (gray), a small difference in the effective voltages applied to the liquid crystal capacitor causes a great change in the gray level. Conversely, when an image signal, which corresponds to gray, is alternately applied to the pixel electrode at the positive polarity and the negative polarity so that the gray levels are adjusted to be approximately the same, the effective voltages applied to the liquid crystal capacitor at both polarities can be equalized. According to the second aspect of the invention, it is preferable that a compensation level corresponding to an image signal at a particular level be a value adjusted so that a gray level difference between a case in which the compensation level is added to the original image signal, and the sum is applied to the pixel electrode, and a case in which an image signal at the other polarity is applied to the pixel electrode becomes small. It thus becomes possible to set a compensation level without taking into consideration the effects of push-down and light leakage.
In order to achieve the foregoing objects, a third aspect of the invention is a liquid crystal display including a plurality of scanning lines; a plurality of data lines; pixels corresponding to intersections between the plurality of scanning lines and the plurality of data lines. The pixels comprised of a pixel electrode and opposing electrode between which liquid crystal is sandwiched to form a liquid crystal capacitor, and a switching element which establishes a connection between the corresponding data line and the pixel electrode in accordance with the level of a signal supplied to the scanning line. An image signal compensation circuit is also provided that compensates for an image signal which is in synchronization with horizontal scanning and vertical scanning before the polarity of the image signal is inverted every predetermined period on the basis of a predetermined constant potential and the image signal is supplied to the pixel electrode through the data line. The image signal compensation circuit includes a compensation-level output unit that outputs a compensation level which corresponds to the level of the uncompensated image signal, and an adder that adds the compensation level to the uncompensated image signal and outputs the sum as the compensated image signal.
According to the third aspect of the invention, as in the first aspect of the invention, application of a DC component to the liquid crystal capacitor is prevented, thereby suppressing deterioration of the display quality due to burn-in.
An electronic apparatus according to the present invention includes the above-described liquid crystal display as a display device. Thus, burn-in is prevented, and high-quality, reliable display can be performed.